Hmm, unfortunately, one of my choices would have been Hunter S. Thompson, but he's no longer available. So I think I might have to go with Stephen King. World leaders need to be able to express themselves well to others, and who would be better at that than a writer?
And of course with Thompson, if talking to them didn't do the job, he was ready, willing and able to get things done whether by sneaking and conniving or by shooting. Both of those can occasionally be handy skills.